Novel reticle design for alternating phase shift mask

ABSTRACT

The present invention provides a method and apparatus for producing 0 degree light and 180 degree phase shifted light having substantially equal intensities as both lights exit an alternating phase shift reticle. A material is inserted within the etched portion of the 180 degree phase shift channel of a reticle, wherein the material contains an index of refraction such that the first order light (+1, −1) is propagated through the 180 degree channel. The end result is a 180 degree phase shifted light having an intensity substantially equal to that of the 0 degree light.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to the field of semiconductormanufacturing. More specifically, it relates to an improved reticledesign for ensuring that both the 0 degree light and the 180 degreelight passing through an alternating phase shift mask are ofapproximately equal amplitude.

[0003] 2. Description of the Related Art

[0004] Lithography processing is a required technology whenmanufacturing conventional integrated circuits (IC's). Many differentlithography techniques exist and all lithography techniques are used forthe purpose of defining geometries, features, lines or shapes onto an ICdie or wafer. In general, a radiation sensitive material, such asphotoresist, is coated over a top surface of a die or wafer and ispatterned using lithography techniques to selectively permit theformation of the desired geometries, features, lines, or shapes.

[0005] One known method of lithography is optical lithography. Thetypical optical lithography process generally begins with the formationof a photoresist layer on the top surface of a semiconductor wafer. Amask typically of quartz and having fully light non-transmissive opaqueregions, which are usually formed of chrome, and fully lighttransmissive clear regions (non-chromed) is then positioned over theaforementioned photoresist coated wafer. Light is directed onto the maskvia a visible light source or an ultra-violet light source. This lightpasses through the clear regions of the mask and exposes the underlyingphotoresist layer, and is blocked by the opaque regions of the mask,leaving that underlying portion of the photoresist layer unexposed. Theexposed photoresist layer is then developed, typically through chemicalremoval of the exposed/non-exposed (depending on whether the photoresistis positive or negative photoresist) regions of the photoresist layer.The end result is a semiconductor wafer coated with a photoresist layerexhibiting a desired pattern. This pattern can then be used for etchingunderlying regions of the wafer.

[0006] In recent years, there has been great demand to increase thenumber of transistors on a given wafer area. Meeting this demand hasmeant that IC designers have had to design circuits with smaller minimumdimensions. However, it was found that the traditional opticallithography process placed real limits on the minimum realizabledimension due to diffraction effects. That is, light shining onto themask is modified as it passes through the mask and therefore the desiredphotoresist pattern differed somewhat from the pattern actuallyachieved. In particular, as the minimum dimension approaches 0.1microns, traditional optical lithography techniques will not work veryeffectively.

[0007] One technique that has been used to realize smaller minimumdevice dimensions is called phase shifting. In phase shifting, thedestructive interference caused by two adjacent clear areas in anoptical lithography mask is used to create an unexposed area on thephotoresist layer. This is accomplished by making use of the fact thatlight passing through the clear regions of a mask exhibits a wavecharacteristic such that the phase of the amplitude of the light exitingfrom the mask material is a function of the distance the light travelsin the mask material. This distance is equal to the thickness of themask material. By placing two clear areas adjacent to each of other on amask, one of thickness t1 and the other of thickness t2, one can obtaina desired unexposed area on the photoresist layer through interference.By making the thickness t2 such that (n−1)(t2) is exactly equal to{fraction (1/2)} λ, where λ is the wavelength of the exposure light, andn is the refractive index of the material of thickness t1, the amplitudeof the light exiting the material of thickness t2 will be 180 degreesout of phase with the light exiting the material of thickness t1.

[0008] Since the photoresist material is responsive to the intensity ofthe light, and the opposite phases of light cancel where they overlap, adark unexposed area will be formed on the photoresist layer at the pointwhere the two clear regions of differing thicknesses are adjacent. Phaseshifting masks are well known and have been employed with variousconfigurations. The configuration described above is known as analternating phase shift mask (APSM). APSMs have been shown to achievedimension resolution of 0.25 microns and below.

[0009] Referring to FIG. 1, a standard reticle design 100 for analternating phase shift mask is depicted. The reticle contains a lighttransmissive region 104 (e.g., quartz) and light non-transmissiveregions 102 (e.g., chrome). The reticle 100 also contains a 0 degreechannel 116 and a 180 degree phase shift channel 114. As describedabove, the 180 degree phase shift channel 114 is created by forming anetched portion 112 in the light transmissive region 104 such that thedistance traveled within the reticle 100 is shorter for light rays 106than for light rays 108, where the light rays 106, 108 are provided byillumination source 110.

[0010] As described above in connection with alternating phase shiftmasks, the difference in the distance traveled by light rays 106, 106through light transmissive region 104 in combination with the refractiveindex of the light transmissive region 104 and the wavelength of thelight rays 106, 108 are calculated such that the amplitude of the lightrays 106 exiting the reticle 100 are 180 degrees out of phase with thelight rays 108 exiting the reticle 100.

[0011] While APSMs have been used with a great degree of success, theystill have some drawbacks. For instance, currently, the amplitude of the0 degree and the 180 degree phase shifted intensities exiting analternating phase shift mask are not equal. For example, FIG. 2 depictsimage intensity for both 0 degree light and 180 degree phase shiftedlight. It is clear from the graphical representation that the intensityof the 180 degree phase shifted light is substantially less (e.g.,approximately 19% less) than the 0 degree light. The reason for thisdiscrepancy is that the first order of diffracted light (designated +1and −1) traveling through the 180 degree phase shift channel 114 doesnot completely pass through the reticle 100 because it is prevented fromdoing so by the light non-transmissive regions 102 (e.g., the chromeregions). Since the intensities of the 0 degree and 180 degree phaseshifted light rays are not equal, the above-described process ofcanceling out opposite polarities of light is not optimized. As aresult, the unexposed areas of the photoresist where the two light raysoverlap are poorly defined, thereby inhibiting optimal IC designs. Thus,a method and apparatus for producing 0 degree light and 180 degree phaseshifted light having substantially equal intensities as that light exitsan alternating phase shifted mask is desirable.

SUMMARY OF THE INVENTION

[0012] The present invention provides a method and apparatus forproducing 0 degree light and 180 degree phase shifted light havingsubstantially equal intensities as that light exits an alternating phaseshifted mask. In the present invention, a material is inserted withinthe etched portion 112 of the 180 degree phase shift charnel 114 of areticle 100, wherein the material contains an index of refraction suchthat the first order light (+1, −1) is propagated through the 180 degreechannel 114. The end result is a 180 degree phase shifted light havingan intensity substantially equal to that of the 0 degree light.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] These and other advantages and features of the invention will bemore clearly understood from the following detailed description of theinvention which is provided in connection with the accompanying drawingsin which:

[0014]FIG. 1 illustrates a standard reticle design for an alternatingphase shift mask;

[0015]FIG. 2 illustrates a graphical representation of respective imageintensities for 0 degree light and 180 degree-phase shifted lightexiting the FIG. 1 reticle;

[0016]FIG. 3 illustrates a reticle design for an alternating phase shiftmask in accordance with a first embodiment of the invention;

[0017]FIG. 4 illustrates a graphical representation of respective imageintensities for 0 degree light and 180 degree phase shifted lightexiting the FIG. 3 reticle;

[0018]FIG. 5 illustrates a reticle design for an alternating phase shiftmask in accordance with a second embodiment of the invention; and

[0019]FIG. 6 illustrates a graphical representation of respective imageintensities for 0 degree light and 180 degree phase shifted lightexiting the FIG. 5 reticle.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0020] Preferred embodiments of the present invention will now bedescribed with reference to FIGS. 3-6. Other embodiments may be realizedand structural, or logical changes may be made to the disclosedembodiments without departing from the spirit or scope of the presentinvention.

[0021]FIG. 3 illustrates a reticle design for an alternating phase shiftmask in accordance with a first embodiment of the invention. The FIG. 3reticle is substantially the same as the FIG. 1 reticle except that theetched portion 112 of the 180 degree channel 114 is filled with amaterial 300 having a depth and an index of refraction such that thecritical angle θc is reached (as will be described below) and the firstorder light (+1, −1) is fully propagated through the 180 degree channel114.

[0022] The critical angle θc is the angle at which there is a state oftotal internal reflection. That is, when light rays (e.g., 106, 108)pass from a first medium (e.g., quartz 104) to a second medium (e.g.,air within etched portion 112) where the second medium is less opticallydense, as the angle of incidence of the light rays is increased,eventually, a critical angle of incidence is reached. At this criticalangle, there is no longer a refracted light ray entering the secondmedium. That is, at the critical angle, there is a refracted light raythat runs parallel with the plane separating the two media. For anglesof incidence greater than the critical angle, there exists totalinternal reflection (i.e., internal to the first medium), whereby nolight travels from the first medium to the second medium. With thisbackground, the depth of the etched portion 112 of the 180 degreechannel 114 (i.e., ρ₁₈₀) and the index of refraction of the fillingmaterial η_(f) can be calculated such that the critical angle θc isreached and the first order of the 180 degree phase shifted light (+1−1) is deflected off of the planes 310, 320 separating the first mediumi.e. material 300 and light transmissive region 104 (e.g., quartz).

[0023] Still referring to FIG. 3, it is known that:

[0024] 1) The depth of the 180 degree phase etched portion${\rho_{180} = \frac{\lambda \quad o}{2\left( {\eta_{q} - \eta_{f}} \right)}};$

[0025] 2) d sin θ=mλ; where d=width of 180 degree channel, m=1 and θ isthe angle of diffraction of first order light (+1, −1); and

[0026] 3)${{\left. 3 \right)\quad {Sin}\quad \theta \quad c} = \frac{\eta_{f}}{\eta_{q}}};$

[0027]  where η_(f)>η_(q) for total internal reflection (as describedabove).

[0028] In addition, it is known that:${{\theta \quad c} = {\frac{\pi}{2} - {\theta \quad d}}};$${{\theta \quad c} = {a\quad {\sin \left( \frac{\eta_{q}}{\eta_{f}} \right)}}};$${{\theta \quad d} = {a\quad \sin \quad \left( \frac{m\quad \lambda}{d} \right)}};\text{and}$$\eta_{f} = {{{\frac{\eta_{q}}{\sin \quad\left\lbrack {\frac{\pi}{2} - {a\quad \sin \quad \left( \frac{m\quad \lambda}{d} \right)}} \right\rbrack}.\text{Therefore,}}\quad \rho_{180}} = \frac{\lambda}{{2\quad \eta_{q}} - \frac{\eta_{q}}{{\sin \quad \frac{\pi}{2}} - {a\quad \sin \quad \frac{m\quad \lambda}{d}}}}}$

[0029] For example, if η_(q)=1.5326, λ=0.248 μm, d=0.520 μm, it iscalculated that η_(f) should be 1.7436 and ρ₁₈₀ should be 0.5875 μm inorder for first order light (+1, −1) to pass through the 180 degreephase shifted channel 114.

[0030] Turning now to FIG. 4, a graphical representation of imageintensities for both the 0 degree light and 180 degree phase shiftedlight exiting the FIG. 3 reticle is depicted. As shown, the intensity ofthe 180 degree phase shifted light is substantially equal to theintensity of the 0 degree light (about a 5% difference).

[0031] Turning now to FIG. 5, a reticle design for an alternating phaseshift mask is depicted, in accordance with a second embodiment of theinvention. The FIG. 5 reticle is substantially the same as the FIG. 3reticle except that in addition to the etched portion 112 of the 180degree channel 114 being filled with material 300, the portion of 180degree channel 114 that extends up to the edge of light non-transmissiveregion 102 is also filled with material 300. In addition, that portionof 0 degree channel 116 that extends up to the edge of lightnon-transmissive region 102 is filled with material 300. The performanceof this embodiment is substantially the same as the performance of theFIG. 3 embodiment.

[0032] Turning now to FIG. 6, a graphical representation of both the 0degree light and 180 degree phase shifted light exiting the FIG. 5reticle is depicted. As shown, the intensity of the 180 degree phaseshifted light is substantially equal to the intensity of the 0 degreelight (about a 4% difference).

[0033] While preferred embodiments of the invention have been describedand illustrated, it should be apparent that many modifications can bemade to the invention without departing from its spirit or scope. Forexample, although the invention has been described with reference toquartz and chrome, respectively, for the light transmissive and lightnon-transmissive regions, any material having similar properties may besubstituted. Accordingly, the invention is not limited by the foregoingdescription or drawings, but is only limited by the scope of theappended claims.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. An alternating phase shift reticle, comprising:a substrate having 0 degree and 180 degree phase shifted light pathstherein; and a material provided in at least one of said 0 degree and180 degree paths which causes an intensity of light exiting said 180degree light path to be substantially equal to an intensity of lightexiting said 0 degree light path.
 2. The reticle of claim 1, whereinsaid substrate is quartz.
 3. The reticle of claim 1, wherein saidmaterial is provided in said 180 degree light path.
 4. The reticle ofclaim 1, wherein said material is provided in both said 0 degree andsaid 180 degree light paths.
 5. The reticle of claim 3, wherein saidmaterial is located within an etched portion of said 180 degree lightpath.
 6. The reticle of claim 5, wherein said material contains apredetermined index of refraction and a predetermined depth.
 7. Thereticle of claim 6, wherein said material is adapted to transmit lightrays from said etched portion of said 180 degree light path to a lightexiting portion of said 180 degree light path.
 8. The reticle of claim7, wherein said material is adapted to direct light rays entering saidetched portion at a predetermined angle such that said light rays exitsaid 180 degree light path.
 9. The reticle of claim 8, wherein saidmaterial is configured to direct said light rays toward a sidewall ofsaid etched portion such that a critical angle is formed, therebyredirecting said light rays in a direction parallel with said sidewall.10. An alternating phase shift reticle, comprising: at least one 0degree optical channel adapted to transit a first set of light rays of afirst intensity; and at least one 180 degree optical channel adapted totransmit a second set of light rays, said second set of light rayshaving a second intensity, wherein said at least one 180 degree channelis at least partially filled with a material having a depth and an indexof refraction such that said first and second intensities aresubstantially equal.
 11. The alternating phase shift reticle of claim10, wherein said at least one 0 degree optical channel is at leastpartially filled with said material.
 12. A method of using analternating phase shift reticle, said method comprising: generatinglight from a light source; receiving first light rays from said lightsource and transmitting them through a 0 degree optical channel of saidreticle, whereupon they exit with a first light intensity; receivingsecond light rays from said light source and transmitting them through a180 degree phase shifted optical channel of said reticle, whereupon theyexit with a second light intensity; and causing said first and secondlight intensities to be substantially equal.
 13. The method of claim 12further comprising changing a direction of a portion of said secondlight rays being transmitted through said 180 degree phase shiftedoptical channel to enable said portion of second light rays to exit said180 degree phase shifted optical channel.
 14. The method as in claim 13further comprising passing said second light rays through a materialhaving a predetermined index of refraction to change said direction ofsaid portion of second light rays.
 15. The method as in claim 14 furthercomprising providing a predetermined depth of said material to changesaid direction of said portion of second light rays.
 16. The method asin claim 15 further comprising changing said direction of said portionof second light rays toward a sidewall of an etched portion of said 180degree phase shifted optical channel such that a critical angle isformed, thereby redirecting said portion of light rays in a directionparallel with said sidewall.
 17. A method of forming an alternatingphase shift reticle, the method comprising: providing a substrate having0 degree and 180 degree phase shifted light paths; and providing amaterial in at least one of said 0 degree and 180 degree paths whichcauses an intensity of light exiting said 180 degree light path to besubstantially equal to an intensity of light exiting said 0 degree lightpath.
 18. The method as in claim 17, wherein said act of providing asubstrate comprises providing a quartz substrate.
 19. The method as inclaim 17, wherein said act of providing a material comprises providing afilling material having a predetermined index of refraction and apredetermined depth within an etched portion of said 180 degree phaseshifted light path.
 20. A method of patterning photoresist, the methodcomprising: shining light from a light source onto an alternating phaseshift reticle located between said photoresist and said light source;and creating a predetermined pattern on said photoresist, saidalternating phase shift reticle further comprising: a substrate having 0degree and 180 degree phase shifted light paths therein; and a materialprovided in at least one of said 0 degree and 180 degree paths whichcauses an intensity of tight exiting said 180 degree light path to besubstantially equal to an intensity of light exiting said 0 degree lightpath.
 21. The method of claim 20, wherein said substrate is quartz. 22.The method of claim 20, wherein said material is provided in said 180degree light path.
 23. The method of claim 20, wherein said material isprovided in both said 0 degree and said 180 degree light paths.
 24. Themethod of claim 22, wherein said material is located within an etchedportion of said 180 degree light path.
 25. The method of claim 24,wherein said material contains a predetermined index of refraction and apredetermined depth.
 26. The method of claim 25, wherein said materialis adapted to transmit light rays from said etched portion of said 180degree light path to a light exiting portion of said 180 degree lightpath.
 27. The method of claim 26, wherein said material is adapted todirect light rays entering said etched portion at a predetermined anglesuch that said light rays exit said 180 degree light path.
 28. Themethod of claim 27, wherein said material is configured to direct saidlight rays toward a sidewall of said etched portion such that a criticalangle is formed, thereby redirecting said light rays in a directionparallel with said sidewall.
 29. A method of transmitting light throughan alternating phase shift reticle, said method comprising: receivinglight rays from a light source and transmitting them through a 0 degreeoptical channel of said reticle; receiving light rays from said lightsource and transmitting them through a 180 degree phase shifted opticalchannel of said reticle; passing first order light of said light raysthrough said 0 degree optical channel; and passing first order light ofsaid light rays through said 180 degree phase shifted optical channel.30. The method of claim 29 further comprising changing a direction of aportion of said light rays being transmitted through said 180 degreeoptical channel to enable said first order light to exit said 180 degreeoptical channel.
 31. The method as in claim 30 further comprisingpassing said light rays passing through said 180 degree optical channelthrough a material having a predetermined index of refraction to changesaid direction of said portion of light rays.
 32. The method as in claim31 further comprising providing a predetermined depth of said materialto change said direction of said portion of light rays.
 33. The methodas in claim 32 further comprising changing said direction of saidportion of light rays toward a sidewall of an etched portion of said 180degree optical channel such that a critical angle is formed, therebyredirecting said portion of light rays in a direction parallel with saidsidewall.